456438 Structure–Function Relationships for Graphene-Supported Pt Nanoclusters

Thursday, November 17, 2016: 4:15 PM
Franciscan C (Hilton San Francisco Union Square)
Hongbo Shi, Chemical Engineering, Univeristy of Massachusetts, Amherst, Amherst, MA, Scott M. Auerbach, Chemistry and Chemical Engineering, University of Massachusetts, Amherst, MA and Ashwin Ramasubramaniam, Mechanical and Industrial Engineering, University of Massachusetts Amherst, Amherst, MA

Platinum-based catalysts play an important role in energy conversion technologies, particularly, in hydrogen-based or methanol-based proton exchange membrane fuel cells (PAFC). Graphene-supported Pt nanoclusters were recently found to be promising catalysts due to their enhanced catalytic activity and tolerance to CO poisoning, as well as their long-term stability toward sintering. However, structure–function relationships that govern the improved electrocatalytic activity in these materials are still not well understood. Here, we employ a combination of empirical potential simulations and density functional theory (DFT) calculations to investigate the structure–function relationships of small Ptn (n=2-80) clusters on model carbon (graphene) supports. A Tersoff-Brenner style empirical potential, was employed within a Genetic Algorithm to investigate the global-minimum structures of Pt clusters in the size range of N=2-80 on pristine as well as defective graphene supports. Point defects in graphene strongly anchor Pt clusters and also appreciably affect geometries of small clusters, which we characterize via various structural metrics such as the gyration radius, average bond length, and average coordination number. Through selected ab initio studies, we find a consistent trend for charge transfer from Pt clusters to defective graphene supports resulting in the lowering of the cluster d-band center. This lowering of the cluster d-band center has been shown previously to result in weaker CO adsorption as well as reduced barriers for CO oxidation. A key finding from the structural analysis is that the fraction of potentially active surface sites in supported clusters is maximized for stable Pt clusters in the size range of 20-30 atoms, which provides a useful design criterion for optimal utilization of the precious metal.

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